The more people that were forced into serving Cairo, the faster it becomes for him to acquire slaves. Look at it like this, two people were abducted and they became slaves
These two people were ordered to acquire more slaves and since they are two people, it was easier to abduct a single mage. After abducting one mage, Cairo's slaves rose to three, and then these three will collaborate with each other to abduct two more slaves
The number increase is not a simple linear graph, the growth is exponential!
BOOM! BOOM! BOOM!
Thousands of loud and powerful explosions occurred everywhere in the Brina Kingdom except to the tens of kilometers around their Royal Palace where the King and their only Archmage was residing